Heretofore, there have been two distinct approaches for achieving high strength pitch-based carbon fibers. One of these approaches features a method of perfecting the chemistry of the pitch precursor, so that the pitch introduced to the spinning process will be highly anisotropic and free from strength-robbing ash and impurities. The theory being that the ultimate product integrity is most dependent upon the chemistry of the precursor. Another approach has been to formulate and process a pitch which would provide the best characteristics for spinning. The theory is that the final product is most influenced by the spinning procedure independent of whether the precursor contains the optimum chemistries.
The present invention is concerned with the latter approach for achieving high strength fibers. While it is realized that it is important to process a pitch precursor to obtain the proper chemistries, the present invention emphasizes the need to focus upon obtaining a precursor having the optimum rheological characteristics required to achieve optimum spinning conditions.
In recent times, there has been much confusion as to the necessary spinning parameters and the rheology of the carbon fiber precursor needed to produce high strength fibers.
It was originally believed that an ordered texture should be produced in the spun pitch in order to align the domains or fibrils such that upon subsequent oxidation and carbonization, these fibrils would link together to form continuous graphite crystallites. The formation of continuous graphite crystallites were believed to be necessary in order to provide the high tensile and mechanical strengths in the fiber. Therefore, the initial wisdom was to provide a spun pitch having a radial texture throughout its cross-section.
It was not long before it was noticed that spun pitch havng a radial cross-section tended to split along the fiber axis, and the high strengths that were theoretically possible were never realized.
More recently, it has been discovered that spun fibers having a random cross-section produce carbon fibers with greater mechanical properties and strengths than the previous radially textured fibers. These fibers do not exhibit the tendency to split along the fiber axis as the previous radially textured fibers.
In order to achieve a random texture in the spun fiber, current carbon precursors are produced having a low glass transition temperature and a low viscosity.
It has not been known in the past, however, what rheology or spin parameters would provide the best results.
The present invention is based upon a mathematical model, which was developed to study the structural changes in the fiber as it is being spun. It was theorized that if one could understand the forces shaping the domains, textures and fibrils during spinning, one would be able to make a better determination of the necessary spinning parameters and rheology needed to effect a strong fiber. The mathematical model was followed by a series of tests designed to affirm or deny the results of the study.
While the complete picture is still not thoroughly understood, the results of the present research have been most illuminating if not actually startling.
It has been discovered that when a precursor is spun and drawn from the counterbored capillaries of the spinnerette, it is acted upon by radial forces tending to influence the shaping of the domains into a radially textured cross-section.
This texture, however, will only be maintained in the final product if the spinning "carrot" of the fiber has a given viscosity as it is being spun and drawn. Changes in the "carrot" viscosity can produce textures in the fiber of all kinds, including: onion skin, radial, random or a hybrid of two or more of the above.
Furthermore, it is theorized that as the viscosity of the "carrot" is varied, the longitudinal alignment of the fibrils will be greatly influenced.
It is noted that a radial texture may form at a particular viscosity of the precursor, wherein the alignment of the fibrils along the longitudinal axis is nearly parallel.
At a higher viscosity, it has been discovered that a radial texture may be formed wherein the alignment of the fibrils along the longitudinal axis is skewed tending to form undulating ribbons in the final fiber product.
According to Reynolds-Sharp theory, the orientation of the mesophase fibrils and the subsequent orientation of the graphite crystallites resulting therefrom after carbonization, should not be parallel or so near parallel, that the fiber becomes susceptible to cracking from internal defects. Expressed in another way, parallel aligned carbon crystallites are more subject to damage from internal defects. These defects are always present in every precursor, and they cannot be eliminated. Therefore, a parallel or near parallel alignment, according to theory should result in a more flaw sensitive fiber and hence, should be avoided.
Our tests have shown that the cracking and splitting of the fibers occurs when alignment of the crystallites tends to parallel the longitudinal axis of the fiber. In other words, the test results appear to conform with theory.
It has been further discovered that as the viscosity of the spinning "carrot" of the fiber is changed, both the texture and the alignment of the fibrils will change, such that it is possible to pass through a spectrum of different textures and alignments. These different spin results appear at present to fall within four distinct zones. In a first zone wheerin a precursor has very low spin viscosities, a fiber with a random texture and crystallites with a high degree of alignment is developed. As the viscosity is increased, a second zone develops wherein a radial textured fiber is formed having crystallites with a lesser degree of alignment.
A third zone is achieved at still higher viscosities wherein the texture becomes random and the alignment of the crystallites become more skewed. A final or fourth zone features a radially textured fiber having crystallites with a highly skewed alignment producing undulating ribbons.
It is believed at this time, that the best precursors are ones that will have a cross-section with an ordered (typically radial) texture and crystallites having a highly skewed alignment with respect to the longitudinal axis such that undulating ribbons are formed in the final fiber product.
It has been discovered that the aforementioned zones are a result of a "spin reversal" in the "carrot" of the spinning pitch. The loss of vorticity and the viscosity at the spin reversal are the two factors which most probably do more to change the texture and alignment characteristics of the fiber than any other factor.
Till now, no one to the best of our knowledge and belief, has realized that such a reversal exists in the spinning "carrot".
At very low viscosities, the vortices in the "carrot" may not form, or may be so weak, that a random texture will form, i.e. the vorticity does not shape the orientation of the fibers. This condition corresponds to zone one, as mentioned above.
As the viscosity increases poorer orientations will be frozen into the surface of the fiber more rapidly and in addition, the spin reversal will act to reorientate the initial radial texture into a second radial texture, i.e. a second zone condition is observed.
When the viscosity of the precursor increases even more, the texture cannot be reformed below the spin reversal thus giving a randomly textured cross-section (zone three).
At sufficiently high enough viscosity, the texture will not be lost at the "spin reversal" and hence, the fiber will maintain its initial radial texture (zone four).
Thus, there is a zone on either end of the viscosity spectrum (zones one and four), which is not influenced by loss of vorticity at the spin reversal. Hence, this zone will provide a preferred fiber texture. At the high viscosity end (zone four), the skewed alignment is such that undulating ribbons in the fiber will result. Thus, the present invention seeks to increase rather than decrease the viscosity of the precursor in order to obtain an optimum rheological condition.